Adapted multiaxial fatigue models based on the critical plane approach to consider the presence of small defects in steel

Abstract

Non-metallic inclusions can often be seen in the fatigue crack initiation site in high-strength steels. These inclusions are known to be detrimental to the fatigue strength, as they behave as stress concentration elements. The same is valid for small defects such as notches, holes and scratches. The goal of this research is to investigate not only the effect of non-metallic inclusions but also of small artificial defects on the fatigue strength of the AISI 4140 steel under multiaxial loading conditions. In order to do so, two critical plane based multiaxial fatigue models are coupled with the √area parameter. This is an empirical parameter, which is used to estimate the uniaxial fatigue strength of metals containing small defects or inclusions. One of the major merits of √area parameter is the fact that the material fatigue limits (under push-pull or torsion) can be estimated by considering only the material hardness and the geometry (area) of the small defect, i.e no costly and lengthy fatigue tests are required. To validate the analysis, experiments under pure and combined push-pull and torsional loading in and out-of-phase have been conducted in smooth and also in specimens containing an artificially produced micro hole. Comparison between the estimates provided by the coupled critical plane-√area parameter multiaxial criterion and data provided very good results, with errors around 20%.